Amphibians continue their “unabated” decline, but researchers have yet to find a “smoking gun” as to why, creating a slippery problem for conservationists.

Writing in the journal Scientific Reports, researchers from the U.S. Geological Survey say that without a clear reason for the decline, coming up with solutions to reverse it will remain elusive.

“Implementing conservation plans at a local level will be key in stopping amphibian population losses, since global efforts to reduce or lessen threats have been elusive,” said Evan Grant, a USGS research wildlife biologist who led the study. “This research changes the way we need to think about amphibian conservation by showing that local action needs to be part of the global response to amphibian declines, despite remaining questions in what is causing local extinctions.”

Researchers did find some causes, but they differ according to region.

For example, “human influence” such as pollution from cities and runoff from agricultural activities were listed as reasons east of the Mississippi River, while in the Upper Midwest and New England, the decline seems tied to disease, particularly a type of fungus.East of the Colorado River, pesticides were listed as a cause, while climate change appears to be hurting amphibians across the Southern U.S. and the West Coast.

Amphibians in the U.S. have been in decline since the 1960s, according to the USGS, even in areas where they are protected, such as national parks. Researchers, using data collected in some locations since 1993, have found that amphibians have been declining in the U.S. at an average rate of 3.8 percent a year.

Declines in some regions were more sharp, opening the possibility that amphibians could completely disappear in certain areas within 20 years.

"Losing 3 or 4 percent of amphibian populations might not sound like a big deal, but small losses year in and year out quickly lead to dramatic and consequential declines,” said USGS ecologist Michael Adams, a study coauthor and the lead for the USGS Amphibian Research and Monitoring Initiative, a group that monitors amphibian populations.

While the study doesn't offer easy answers for how to stem the decline of amphibians, narrowing down the causes by region could be the first step in sparking local action to save the creatures.